Scalable video coding system with parameter signaling

ABSTRACT

A method is provided for encoding a digital video to provide for improved color mapping. The digital video has values in a first color space, and the method includes performing a color mapping operation on values in each sub-picture to convert the values in the first color space to values in a second, narrower, color space, wherein the color mapping operation is adapted based on the content of each sub-picture, encoding the values in the second color space into a base layer, performing a reverse color mapping operation on decoded values from the base layer in the second color space in each sub-picture to generate a reconstructed reference frame having values in the first color space, encoding values in the first color space into an enhancement layer based at least in part on the reconstructed reference frame, combining the base layer and the enhancement layer into a bitstream, sending the bitstream to a decoder, and sending one or more parameters to the decoder that describe the adaption of the reverse color mapping operation for at least some sub-pictures.

CLAIM OF PRIORITY

This application claims priority under 35 U.S.C. § 119(e) from earlierfiled U.S. Provisional Application Ser. No. 62/150,476, filed Apr. 21,2015, which is hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to the field of video encoding anddecoding, particularly a method of signaling parameters forreconstructing reference pictures at a decoder in a scalable videocoding system.

BACKGROUND

High Dynamic Range (HDR) video and Wide Color Gamut (WCG) video offergreater ranges of luminance and color values than traditional video. Forexample, traditional video can have a limited luminance and color range,such that details in shadows or highlights can be lost when images arecaptured, encoded, and/or displayed. In contrast, HDR and/or WCG videocan capture a broader range of luminance and color information, allowingthe video to appear more natural and closer to real life to the humaneye.

Although the extended range of values possible in HDR and/or WCG videocan better approximate real life, many monitors cannot yet display sucha large range of values. Although access to HDR monitors is improving,there is fragmentation between the reproducibly color ranges ondifferent types of monitors. While it is possible to encode one versionof a piece of content for non-HDR monitors and encode another for HDRmonitors, encoding and transmitting two different versions of abitstream can be time-consuming and inefficient.

Some systems have been developed that can include non-HDR and HDRinformation within different layers of the same bitstream, such that adecoding device can ignore the HDR layer if it is not connected to amonitor that can reproduce the color information in that layer. Forexample, the Scalable Video Coding (SVC) extension of the MPEG-4Advanced Video Coding (AVC) coding scheme can handle bitstreams with abase layer of non-HDR information and an enhancement layer withadditional information related to HDR values. However, such existingsystems generally use the base layer information to predict informationin the enhancement layer in a static way that is unrelated to thecontent of the video.

What is needed is a scalable video coding system where an encoder canapply different operations to different pictures or sub-pictures basedon the content of the picture before encoding the enhancement layer, andsend parameters to decoders that indicate an appropriate operation touse when decoding the enhancement layer.

SUMMARY

The present disclosure provides a method of encoding a digital video,the method comprising receiving a digital video at a video encoder, thedigital video comprising values in a first color space, performing acolor mapping operation on values in each sub-picture at the videoencoder to convert the values in the first color space to values in asecond color space that is narrower than the first color space, whereinthe video encoder adapts the color mapping operation based on thecontent of each sub-picture, encoding the values in the second colorspace into a base layer, decoding and performing a reverse color mappingoperation on the values in the second color space in each sub-picture asdecoded from the base layer to generate a reconstructed reference framehaving values in the first color space, encoding the values in the firstcolor space into an enhancement layer based at least in part on thereconstructed reference frame, combining the base layer and theenhancement layer into a bitstream, sending the bitstream to a decoder,and sending one or more parameters to the decoder that describe theadaption of the reverse color mapping operation for at least somesub-pictures.

The present disclosure also provides a method of decoding a digitalvideo, the method comprising receiving a bitstream comprising a baselayer and an enhancement layer at a video decoder, receiving one or moreparameters associated with at least some sub-pictures, decoding baselayer values in a first color space from the base layer, performing areverse color mapping operation on the base layer values within eachsub-picture, to generate a reconstructed reference picture having valuesin a second color space that is wider than the first color space,wherein the video decoder adapts the reverse color mapping operation foreach sub-picture based on received parameters associated with thatsub-picture, and decoding enhancement layer values in the second colorspace from the enhancement layer using prediction based on thereconstructed reference picture.

The present disclosure also provides a video encoder comprising a datatransmission interface configured to receive a digital video comprisingfull resolution values, and a processor configured to perform adownsampling operation to convert the full resolution values intodownsampled values, encode the downsampled values into a base layer,decode the base layer into reconstructed downsampled values, perform anupsampling operation on the reconstructed downsampled values to generatea reconstructed full resolution reference frame for a particular codinglevel, encode the full resolution values into an enhancement layer basedat least in part on the reconstructed full resolution reference frame,and combine the base layer and the enhancement layer into a bitstream,wherein the data transmission interface is further configured to sendthe bitstream to a decoder, and send one or more parameters to thedecoder that describe the upsampling operation for the particular codinglevel.

The present disclosure also provides a video decoder comprising a datatransmission interface configured to receive a bitstream comprising abase layer and an enhancement layer, and one or more parametersassociated with an upsampling operation for a particular coding level,and a processor configured to derive the upsampling operation for theparticular coding level from the one or more parameters, decode the baselayer into values at a downsampled resolution, perform the upsamplingoperation to generate a reconstructed reference picture at a fullresolution, and decode a picture in the enhancement layer using thereconstructed reference picture.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details of the present invention are explained with the help ofthe attached drawings in which:

FIG. 1 depicts an embodiment of a scalable video coding systemcomprising an encoder and a decoder.

FIG. 2 depicts a flowchart of a process for encoding an HDR input videousing a scalable video coding system.

FIG. 3 depicts a first exemplary process for generating cross-layerinformation from a base layer at an encoder.

FIG. 4 depicts a second exemplary process for generating cross-layerinformation from a base layer.

FIG. 5 depicts a flowchart of a process for decoding an HDR decodedvideo from a bitstream using a scalable video coding system.

DETAILED DESCRIPTION

FIG. 1 depicts an embodiment of a scalable video coding systemcomprising an encoder 100 and a decoder 102. An encoder 100 can compriseprocessors, memory, circuits, and/or other hardware and softwareelements configured to encode, transcode, and/or compress input video104 into a coded bitstream. The encoder 100 can be configured togenerate the coded bitstream according to a video coding format and/orcompression scheme, such as HEVC (High Efficiency Video Coding),H.264/MPEG-4 AVC (Advanced Video Coding), or MPEG-2. By way of anon-limiting example, in some embodiments the encoder 100 can be a Main10 HEVC encoder.

The encoder 100 can receive an input video 104 from a source, such asover a network or via local data storage from a broadcaster, contentprovider, or any other source. The encoder 100 can encode the inputvideo 104 into the coded bitstream. The coded bitstream can betransmitted to decoders 102 over the internet, over a digital cabletelevision connection such as Quadrature Amplitude Modulation (QAM), orover any other digital transmission mechanism.

A decoder 102 can comprise processors, memory, circuits, and/or otherhardware and software elements configured to decode, transcode, and/ordecompress a coded bitstream into decoded video 106. The decoder 102 canbe configured to decode the coded bitstream according to a video codingformat and/or compression scheme, such as HEVC, H.264/MPEG-4 AVC, orMPEG-2. By way of a non-limiting example, in some embodiments thedecoder 102 can be a Main 10 HEVC decoder. The decoded video 106 can beoutput to a display device for playback, such as playback on atelevision, monitor, or other display.

In some embodiments, the encoder 100 and/or decoder 102 can be adedicated hardware devices. In other embodiments the encoder 100 and/ordecoder 102 can be, or use, software programs running on other hardwaresuch as servers, computers, or video processing devices. By way of anon-limiting example, an encoder 100 can be a video encoder operated bya video service provider, while the decoder 102 can be part of a set topbox connected to a television, such as a cable box.

The input video 104 can comprise a sequence of pictures, also referredto as frames. In some embodiments, colors in the pictures can bedescribed digitally using one or more values according to a color spaceor color model. By way of a non-limiting example, colors in a picturecan be indicated using an RGB color model in which the colors aredescribed through a combination of values in a red channel, a greenchannel, and a blue channel. By way of another non-limiting example,many video coding formats and/or compression schemes use a Y′CbCr colorspace when encoding and decoding video. In the Y′CbCr color space, Y′ isa luma component while Cb and Cr are chroma components that indicateblue-difference and red-difference components.

In some embodiments or situations, the input video 104 can be an HDRinput video 104. An HDR input video 104 can have one or more sequenceswith luminance and/or color values described in a high dynamic range(HDR) and/or on a wide color gamut (WCG). By way of a non-limitingexample, a video with a high dynamic range can have luminance valuesindicated on a scale with a wider range of possible values than anon-HDR video, and a video using a wide color gamut can have its colorsexpressed on a color model with a wider range of possible values in atleast some channels than a non-WCG video. As such, an HDR input video104 can have a broader range of luminance and/or chroma values thanstandard or non-HDR videos.

In some embodiments, the HDR input video 104 can have its colorsindicated with RGB values in a high bit depth format, relative tonon-HDR formats that express color values using lower bit depths such as8 or 10 bits per color channel. By way of a non-limiting example, an HDRinput video 104 can be in an EXR file format with RGB color valuesexpressed in a linear light RGB domain using a 16 bit floating pointvalue for each color channel.

As shown in FIG. 1 , the coded bitstream generated by encoder 100 andtransmitted to the decoder 102 can comprise a base layer 108 and anenhancement layer 110. By way of a non-limiting example, scalable videocoding schemes such as HEVC and AVC with a Scalable Video Coding (SVC)extension can encode and decode bitstreams comprising a base layer 108and an enhancement layer 110. The base layer 108 and the enhancementlayer 110 can include information about an HDR input video 104 atdifferent quality levels, different color spaces, different spatialresolutions, and/or different bit depths. By way of a non-limitingexample, the enhancement layer 110 can be encoded with a wider colorgamut, higher bit-depth, and/or a higher spatial resolution than thebase layer 108.

In some embodiments, the base layer 108 can include information aboutnon-HDR and/or non-WCG components of an HDR input video 104, while theenhancement layer 110 can include additional information about extendedranges of color values not described by the base layer 108. By way of anon-limiting example, the base layer 108 can be encoded to include colorvalues from the HDR input video 104 that are within a range of colorsthat can be displayed on a standard non-HDR monitor, while theenhancement layer 110 can be encoded with information about additionalcolor values that are beyond the base layer's range, such that HDRmonitors configured to display a wider range of color values can use theinformation in the enhancement layer 110 to display an HDR decoded video106. As such, the base layer 108 can include a subset of the full rangeof colors described in the original HDR input video 104, while theenhancement layer 110 can describe the full range of colors from theoriginal HDR input video 104 in combination with the base layer 108.

FIG. 2 depicts a flowchart of a process for encoding an HDR input video104 using a scalable video coding system.

At step 202, the encoder 100 can receive an HDR input video 104 from asource. The HDR input video 104 can have full resolution color values,such as values in an HDR and/or WCG range, in a particular color space,at a high bit depth, at a high spatial resolution, and/or in any otherformat denoted as full resolution.

At step 204, the encoder 102 can perform one or more downsamplingoperations on color values from the HDR input video 104, to convert themfrom full resolution to a downsampled resolution.

One downsampling operation can be a color space conversion of the valuesinto a non-HDR color space. In some embodiments, color space conversioncan take a triplet sample, such as red, green, and blue components in anRGB color domain, and map it to a corresponding sample at the samespatial location in another color space. By way of a non-limitingexample, when the HDR input video 104 has color values in a wide colorgamut space, such as the DCI-P3 or BT.2020 color space, the encoder 100can convert the color values to a narrower color space, such as theBT.709 color space.

Another downsampling operation can be reducing the bit depth of the fullresolution values. By way of a non-limiting example, in some embodimentsor situations the full resolution values can be expressed with 16-bitfloating point values, and a downsampling operation can convert them toa format with a lower bit depth, such as 8 or 10-bit values.

Still another downsampling operation can be to reduce the spatialresolution of the full resolution values. By way of a non-limitingexample, the full resolution values can describe pictures in a 4Kresolution, and the downsampling operation can generate values for lowerresolution versions of those pictures, such as a 1080p resolution.

At step 206, the encoder 100 can encode pictures described by thedownsampled values into the base layer 108. In some embodiments, thepixels of each picture can be broken into sub-pictures, such asprocessing windows, slices, macroblocks in AVC, or coding tree units(CTUs) in HEVC. The encoder 100 can encode each individual pictureand/or sub-picture using intra-prediction and/or inter-prediction.Coding with intra-prediction uses spatial prediction based on othersimilar sections of the same picture or sub-picture, while coding withinter-prediction uses temporal prediction to encode motion vectors thatpoint to similar sections of another picture or sub-picture, such as apreceding or subsequent picture in the input video 104. As such, codingof some pictures or sub-pictures can be at least partially dependent onother reference pictures in the same group of pictures (GOP).

At step 208, the encoder 100 can encode pictures described by the HDRinput video's original full resolution values into the enhancement layer110. The encoder 100 can encode the enhancement layer using, in part,cross-layer information 114 indicating how to reconstruct or predictfull resolution values from the downsampled values of the base layer108. The cross-layer information 114 can comprise reference picturesdecoded and upsampled from the base layer 108, and/or parameters 112 ofa function such as a color mapping operation, a filter operation, and/ora coding transfer function that can predict reconstructed fullresolution values for the enhancement layer 110 from downsampled valuesdecoded from the base layer 108.

At step 210, the encoder 100 can combine the base layer 108 andenhancement layer 110 into a bitstream that can be transmitted to adecoder 102. The decoder 102 can decode the bitstream as described belowwith respect to FIG. 5 .

FIG. 3 depicts a non-limiting example of a process for generatingcross-layer information 114 from a base layer 108 at the encoder 100.After the base layer 108 is encoded at step 206, the encoder 100 canperform a decoding operation on the base layer 108 to obtain downsampledvalues. The encoder 100 can then perform one or more upsamplingoperations 302 on the downsampled values to reconstruct a referencepicture described by full resolution values. Upsampling operations 302can include a reverse color mapping operation, increasing the bit depth,and/or increasing the spatial resolution. The upsampling operations 302can be selected or adjusted at the encoder 100, such that reconstructedfull resolution values approximate the original full resolution valuesfrom the HDR input video 104 and an error metric describing differencesbetween the reconstructed and original full resolution values, such as amean-squared error (MSE) or peak signal to noise ratio (PSNR), isminimized. As will be described below with respect to FIG. 5 , thedecoder 102 can also perform a substantially similar decoding operationon the base layer 108 and one or more substantially similar upsamplingoperations 302 during its decoding process.

The reconstructed reference pictures generated with the upsamplingoperations 302 can be used during step 208 when encoding full resolutionvalues from the HDR input video 104 into the enhancement layer. By wayof a non-limiting example, pictures in the HDR input video 104 can bespatially predicted for the enhancement layer 110 based on fullresolution reference pictures reconstructed from the base layer 108.

FIG. 4 depicts another non-limiting example of a process for generatingcross-layer information 114 from a base layer 108. In this example,cross-layer information can describe a filter through which base layer108 values can be converted into full resolution values for predictingthe enhancement layer 110.

After the base layer 108 is encoded at step 206, the encoder 100 canselect a set of input samples from the base layer 108, at thedownsampled resolution. By way of a non-limiting example, the inputsamples can be a two-dimensional subset of samples taken at thedownsampled resolution.

At step 404, the encoder 100 can select an appropriate filter that canconvert the input samples at the downsampled resolution to reconstructedfull resolution values. By way of a non-limiting example, when the setof input samples is a set of two dimensional samples at the downsampledresolution, the encoder 100 can select a two dimensional filter matchthat corresponds to characteristics of the set of two dimensionalsamples.

At step 406, the selected filter can be applied to the set of inputsamples, to produce reconstructed values at full resolution.

By way of a non-limiting example, when filtering is separable, a filterh[n; m] can be applied along rows and columns of the set of inputsamples at the downsampled resolution to produce an output values y[m]at full resolution for each output index m. In some embodiments, theencoder 100 can have a set of M filters, and at each output index m aparticular filter h from the set can be chosen based on can be chosenfrom the set of M filters, as defined by h[n; m mod M]. In someembodiments filters h[n; p], where p=m mod M, can correspond to filterswith M different phase offsets, such as where p=0, 1, . . . , M−1 whenthe phase offset is p/M.

In still other embodiments, the cross-layer information 114 can describetransfer function mappings through which enhancement layer 110 valuescan be predicted from base layer 108 values. By way of non-limitingexamples, transfer function mappings can be mappings between downsampledvalues and full resolution values according to a gamma function, aperceptual quantizer (PQ) function, or a piecewise function such as apiecewise linear function.

FIG. 5 depicts a flowchart of a process for decoding an HDR decodedvideo 106 from a bitstream using a scalable video coding system.

At step 502, the decoder 102 can receive a bitstream generated by anencoder 100. The bitstream can comprise a base layer 108 describingvalues at a downsampled resolution, and an enhancement layer 110 that incombination with the base layer 108 can describe values in a fullresolution, such as values in an HDR and/or WCG range, in a particularcolor space, at a high bit depth, at a high spatial resolution, and/orin any other format denoted as full resolution.

At step 502, the decoder 102 can decode the base layer 108 to obtaindownsampled values. If the decoder 102 is outputting video to a monitoror other device that only needs the video in the downsampled resolution,it can ignore the enhancement layer 110 and output a non-HDR decodedvideo 106 using those downsampled values at step 506.

However, if the decoder 102 is outputting video to a monitor or otherdevice that can playback or use values in the full resolution, thedecoder 102 can use cross-layer information 114 from the base layer 108to also decode the enhancement layer 110 at step 508. Reconstructed fullresolution values decoded from the enhancement layer 110 can be outputas HDR decoded video 106 at step 510.

By way of non-limiting examples, to decode inter-predicted pictures inthe enhancement layer 110, the decoder 102 can decode downsampled valuesfrom the base layer 108 at step 504, then perform one or more upsamplingoperations 302 as described above to generate reconstructed referencepictures at the full resolution. The full resolution reconstructedreference pictures can be used as cross-layer information 114 duringstep 508 to decode inter-predicted pictures in the enhancement layer110.

As described above, full resolution values in the enhancement layer 110can be encoded and decoded at least in part based on predictions of fullresolution values generated from downsampled values in the base layer108. Accordingly, when the base layer 108 and enhancement layer 110 arecombined into a bitstream and sent to a decoder, the encoder 100 canalso send one or more parameters 112 that can indicate to the decoder102 how to upsample values from the base layer 108 into referencepictures at the full resolution, such that they can be used whendecoding spatially predicted pictures in the enhancement layer 110. Byway of non-limiting examples, the parameters 112 can be values sent fromthe encoder 100 that a decoder 102 can use to derive an upsamplingoperation such as a color mapping operation, a filter, or a transferfunction. The decoder 102 can thus determine an appropriate upsamplingoperation for sets of downsampled values from the base layer 108 toconvert values from the base layer 108 into full resolution values thatcan reconstruct reference pictures the decoder 102 can use when decodingthe enhancement layer 110.

The encoder 100 can send a set of one or more parameters 112 describingupsampling operations 302 between for different positions within thesame picture, for a single picture, and/or for one or more sequences ofpictures, such as GOPs. By way of a non-limiting example, the parameters112 can be different color mapping operations to use for differentregions within the same frame, such as sub-pictures including processingwindows, slices, macroblocks, or CTUs. The decoder 102 can use theparameters 112 to derive an appropriate upsampling operation for asub-picture, picture, or supra-picture sequence.

In some embodiments, the encoder 100 can send parameters 112 at asub-picture, picture, or supra-picture coding level such that a decoder102 can derive an appropriate upsampling operation 302 that can assistin reconstructing a reference picture at full resolution for that codinglevel. By way of a non-limiting example, when the decoder 102 receivesparameters 112 through which it can derive a color mapping operation fora reference frame, the decoder 102 can keep the reference frame and theparameters for the that reference frame at one or more resolutions, suchas 4×4, 8×8, or 16×16. As such, it can re-use the reference frame and/orreceived parameters as appropriate when decoding pictures from theenhancement layer 110 that were predicted based on that reference frame.In some embodiments, the decoder 102 can adjust received parameters 112based on the desired spatial resolution, such as 2×2×2, 1×1×1, or 8×2×2color mapping parameters.

In some embodiments, when the decoder 102 receives parameters relevantto some spatial locations within a frame, it can predict parameters 112for other spatial locations based on spatial prediction. By way of anon-limiting example, the decoder 102 can predict parameters 112 for aparticular location based on parameters 112 received from the encoder100 for neighboring locations.

In some embodiments, when the decoder 102 decodes the enhancement layer110 using temporal prediction, a picture or sub-picture can be decodedbased on parameters 112 received for collated pixels of a referencepicture.

In some embodiments, when parameters 112 are received for a particularreference picture within a GOP, those parameters 112 can be used forother pictures within the GOP and/or other GOPs, until new parameters112 are received.

In some embodiments or situations, the encoder 100 can send parameters112 to the decoder 102 on a supra-picture level. In these embodiments orsituations, the upsampling operation 302 described by the parameters 112can be applicable to all the pictures in a given sequence, such as aGOP. In some embodiments, the encoder 100 can send the parameters 112 tothe decoder 102 on a supra-picture level using supplemental enhancementinformation (SEI) message. In other embodiments, the encoder 100 cansend the parameters 112 to the decoder 102 on a supra-picture levelusing video usability information (VUI) or other information within aSequence Parameter Set (SPS) associated with the GOP. In someembodiments, the decoder 102 can use the most recently receivedparameters 112 until new parameters 112 are received, at which point itcan derive a new upsampling operation 302 from the newly receivedparameters 112. By way of a non-limiting example, parameters 112 caninitially be set in an SPS, and then be updated on a per-GOP basis asthe characteristics of the input video 104 changes.

In some embodiments or situations, the encoder 100 can send parameters112 to the decoder 102 on a picture level. In these embodiments orsituations, the upsampling operation 302 described by the parameters 112can be applicable to full pictures. In some embodiments, the encoder 100can send the parameters 112 to the decoder 102 on a picture level withina Picture Parameter Set (PPS) associated with a picture.

In some embodiments, such as when the pictures are P or B pictures thatwere encoded with reference to one or more reference pictures, thedecoder 102 can receive and maintain parameters 112 for the referencepictures, as well as parameters 112 specific to individual temporallyencoded pictures. As such, when the decoder 102 previously generated areference picture with full resolution values using a first set ofparameters 112, and the decoder 102 receives different parameters 112for decoding a P or B picture encoded with reference to the referencepicture, the decoder 102 can first reverse an upsampling operation 302it previously performed on the reference picture using the parameters112 received for the reference picture to return it to downsampledvalues. The decoder 102 can then perform a new upsampling operation 302on the reference picture's downsampled values using a second set ofparameters 112 received for the current picture, to re-map the referencepicture in full resolution values using the current picture's parameters112. The re-mapped reference picture can then be used in decoding thecurrent picture and predicting its values in the enhancement layer 110.In some embodiments, the decoder 102 can re-map reference picturesaccording to new parameters 112 associated with a current picture if thenew parameters 112 differ from old parameters 112 associated with thereference picture. In alternate embodiments, the decoder 102 can re-mapreference pictures as described above if re-mapping is indicated in aflag or parameter received from the encoder 100.

In some embodiments or situations, the encoder 100 can send parameters112 to the decoder 102 on a sub-picture level. In these embodiments orsituations, the upsampling operation 302 described by the parameters 112can be applicable to sub-pictures within a picture, such as processingwindows, slices, macroblocks, or CTUs.

In some embodiments, the decoder 102 can receive and maintain parameters112 for a current sub-picture and all reference pictures orsub-pictures, such as pixel blocks of size 4×4 or 8×8. As such, whendecoding a sub-picture that was coded with reference to one or morereference pictures, the decoder 102 can first reverse previousupsampling operations 302 performed on reference pixels using parameters112 previously received for the reference pixels to return it todownsampled values. The decoder 102 can then apply a new upsamplingoperation 302 on the reference pixels using new parameters associatedwith the current sub-picture to re-map the reference pixels into fullresolution values, such that the decoder 102 can decode the currentsub-picture's enhancement layer values using the re-mapped sub-pixels.In some embodiments, the decoder 102 can re-map reference pixelsaccording to new parameters 112 associated with a current sub-picture ifthe new parameters 112 differ from old parameters 112 associated withthe reference pixels. In alternate embodiments, the decoder 102 canre-map reference pixels as described above if re-mapping is indicated ina flag or parameter received from the encoder 100.

Although the present invention has been described above withparticularity, this was merely to teach one of ordinary skill in the arthow to make and use the invention. Many additional modifications willfall within the scope of the invention, as that scope is defined by thefollowing claims.

The invention claimed is:
 1. A method of decoding a digital videosignal, comprising: receiving in a video decoder a bitstream containingthe digital video signal, the bitstream comprising a base layer and anenhancement layer; receiving one or more sets of parameters, eachassociated with a respective one of a plurality of sub-pictures in thebitstream, each of the subpictures having a different spatial positionwithin a picture relative to other said subpictures; decoding base layervalues in a first color space in said base layer; within each of thesub-pictures, performing a reverse color mapping operation on said baselayer values within each respective subpicture using the set ofparameters respectively associated with the respective sub-picture, togenerate a reconstructed reference picture having values in a secondcolor space wider than said first color space, wherein said reversecolor mapping operation for each respective subpicture maps a color inthe first color space of the base layer to a color in the second colorspace of the base layer, wherein the mapping of a color in the firstcolor space to a color in the second color space differs between atleast two of the plurality of sub-pictures; and decoding enhancementlayer values in said second color space in said enhancement layer usingprediction based on said reconstructed reference picture and providingthe decoder output to a video player; wherein the decoding of theenhancement layer comprises decoding a current sub-picture withreference to the re-mapped sub-pictures output.
 2. The method of claim1, further comprising predicting parameters for sub-pictures for whichparameters were not received, based on received parameters associatedwith neighboring sub-pictures.